Deep drawing is understood to mean the shaping of a sheet metal section (circular blank, plate blank) into a hollow body, or the shaping of a hollow body into a hollow body with a smaller circumference, with or without intentional modification of the sheet metal thickness. During shaping, segments of the blank must be folded up on the cylinder wall, with the parts inbetween being displaced, thus creating radial tensile and tangential compressive stresses. Bending occurs when the cutting bell runs over the blank. The blank holder impinged on by force is provided to prevent the radially outwardly projecting flange from buckling and forming folds under the influence of the tangential compressive stresses.
The caps provided for glass bottles are drawn from relatively thin metal sheets. These sheets, in particular when used in the food industry, are enameled and in many cases have commercial printing. A problem with the cap manufacture is that during shaping in the axially outer region of the cup the enameling cracks and forms fine colored filaments. The cracked enameling does not detract from the appearance, since the axial end of the blank is subsequently rolled in so that the cracked locations, which are also very fine, are not visible on the finished product. However, the colored filaments become lodged not only in the tools but also on the edge of the blank, and a cotton-like texture forms in the tools which must be regularly removed. In addition, the filaments that remain on the cup, in particular on the edge thereof, must be carefully removed so that during filling they do not come into contact with the filling material (foodstuffs), which would be unacceptable.
Caps for glass bottles are mass-produced articles which are manufactured in large quantities in a tool with high cycling times. The cycling times typically have values of approximately 300 min−1. To remove the filaments, the manufacturing unit containing the tool must be shut down and blown out or cleaned, thereby lengthening the production time and also increasing the manufacturing costs. The filaments must be blown out very carefully so that the operators of the unit are not subjected to health risks. In addition, the room in which the units are set up must be continually cleaned to remove the colored filaments.
Various methods having stepwise deformation have been developed in the prior art. DD 233 036 A3 discloses a method for deep-drawing sheet metal parts in which a first draw is followed by a second draw within the same press stroke in the same direction, the drawing force of the first draw being employed as a hold-down force by the second draw and being reduced with progressive drawing depth. The hold-down force is progressively reduced from the start of the second draw, and is entirely eliminated before the shaping in the second draw is completed.
In addition, a drawing method for a disk-like metal sheet is known from DE 692 06 748 T2 in which an annular holding element and/or the drawing tool, which have a residual flange region, are moved in such a way that the holding operation is terminated immediately before the drawing stage is completed. The flange region is then drawn while the back end of the flange region is released.
Both methods share the common feature that they are each carried out in a single tool, so that the shaped part remains in this tool during the stepwise deformation. In practical operation, in particular for high cycling times, release of the hold-down force cannot be reliably ensured. For example, for a pneumatically operated unit the response time for the control is too slow to maintain the high cycling time. For this reason, filament or tail formation cannot be ruled out, even during the second process step when the blank holder should not exert any force on the blank to be deformed.